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Investigating the role of muscle physiology and spinal circuitry in sensorimotor control

INVESTIGATING THE ROLE OF MUSCLE PHYSIOLOGY AND SPINAL
CIRCUITRY IN SENSORIMOTOR CONTROL
by
George A. Tsianos
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(BIOMEDICAL ENGINEERING)
December 2012
Copyright 2012 George A. Tsianos

Making voluntary movements requires proper recruitment of muscles that exert torques at the joints. The brain controls these torques indirectly by sending commands to the spinal circuitry, which continuously integrates them with proprioceptive feedback and recurrent projections from motoneurons. The nature of this transformation and its implications for motor control have been investigated by building a realistic model of the lower motor system (spinal circuitry plus musculoskeletal system) and determining how much of the dynamics of reaching movement it can generate entirely on its own. ❧ An oversimplified model of the brain was employed in order to force the lower motor system to generate all of the necessary dynamics. Its outputs, representing supraspinal control of fusimotor gain, interneuronal biasing activity and presynaptic inhibition/facilitation, controlled a realistic set of spinal circuits based on the classical interneuronal types (propriospinal, monosynaptic Ia-excitatory, reciprocal Ia-inhibitory, Renshaw inhibitory and Ib-inhibitory pathways). Commands to the spinal circuitry were unmodulated step functions whose amplitudes were trained using a simple optimization algorithm and a cost function. ❧ Despite the large number of control points in the spinal cord model (greater than 400 for our six-muscle model) and the oversimplified descending inputs, it was surprisingly easy to train the system to perform motor tasks such as resisting an impulsive perturbation applied to the endpoint and center-out reaches to multiple directions along with the complex muscle dynamics required to achieve them. This is because the high-dimensional space to be controlled appears to have many ""good enough"" solutions and relatively few undesirable local minima. Initially, energetics were not part of the performance criteria; nevertheless, the emerging strategies of muscle recruitment in those solutions were often metabolically efficient. Incorporating energetics into the cost function further improved the efficiency while maintaining acceptable kinematic behavior. ❧ It was also shown that solutions to new tasks (e.g. having untrained durations directions and distances) can be interpolated from solutions to tasks learned previously. Such generalization of solutions improves the rate of learning in many situations and also reduces the storage capacity required for the brain to memorize motor repertoires. ❧ These results suggest that the genetically specified circuitry of the spinal cord may have evolved to permit rapid and reliable solutions to new sensorimotor problems and that it tends to facilitate solutions with low energetic cost. These properties of spinal circuitry must be considered when assigning functionality to the motor control centers of the brain.

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INVESTIGATING THE ROLE OF MUSCLE PHYSIOLOGY AND SPINAL
CIRCUITRY IN SENSORIMOTOR CONTROL
by
George A. Tsianos
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(BIOMEDICAL ENGINEERING)
December 2012
Copyright 2012 George A. Tsianos